The present invention relates to a rolling bearing, and, more particularly, a rolling bearing which is used which is used under a special environment, for example, under an environment requiring corrosion resistance to water content, sea water and chemicals, e.g., in a food machine, a semiconductor producing apparatus and a chemical fiber producing machine, or in a tool machine which operates at a high rotary speed.
As a bearing which must be corrosion-resistant there has been heretofore used relatively often a sliding bearing made of a material having an excellent corrosion resistance. In recent years, rolling bearings have been used more and more from the standpoint of torque reduction that prevents dynamic loss or eliminates the necessity of maintenance and improvement of product quality.
As the material for such rolling bearings there is mostly used a low-alloy steel such as two kinds of high carbon chromium bearing steels (SUJ2) and case hardening steel (SCR420). However, rolling bearings are used in various working conditions. Thus, if such a rolling bearing made of a low-alloy steel is used under environmental conditions which can be contaminated by water content or sea water, the contamination by even a slight amount of water content or sea water corrodes the bearing portion thereof corrodes with rust that disables the rolling bearing from working. Thus, martensite stainless steel having an excellent corrosion resistance and a high chromium content (e.g., SUS440C) is used under such environmental conditions.
However, a rolling bearing comprising races and rolling elements both of which are made of martensite-based stainless steel (hereinafter simply referred to as xe2x80x9cstainless steelxe2x80x9d) can exhibit an insufficient corrosion resistance in some working atmospheres. In this case, corrosion occurs with chromium-deficient layer in the vicinity of coarse eutectic carbide as a starting point to reduce precision such as surface smoothness, possibly making it impossible to secure the desired bearing life. In particular, a rolling bearing adapted for use in semiconductor producing apparatus, etc. is subject to attack by a corrosive gas or chemical that can corrode stainless steel. Thus, it is required that such a rolling bearing comprise a material having a better corrosion resistance than stainless steel.
From this standpoint of view, as a bearing material constituting a rolling bearing adapted for use in corrosive working atmospheres there has heretofore been used a ceramic material such as silicon nitride (Si3N4) (hereinafter referred to as xe2x80x9cfirst conventional techniquexe2x80x9d).
In the machine tool industry, on the other hand, the recent trend is for more machines to operate at higher rotary speed. To this end, it is required for the rolling bearing for supporting the rotary portion of machine tools to have higher precision and withstand severer working conditions. When a machine tool operates at a raised rotary speed, the so-called bearing clearance is reduced, causing further rolling friction that adds to heat generation. As a result, the temperature of the bearing rises.
The rise in the heat generation due to rolling friction is considered to be attributed to the rise in the centrifugal force applied to the rolling elements. In order to lessen the centrifugal force and hence lower the temperature of the rolling elements, a rolling bearing comprising rolling elements made of ceramic material, which exhibit a small density (specific gravity), rather than low-alloy steel has heretofore been put into practical use. However, with the recent trend for more machine tools to operate at even higher rotary speed, mere reduction of the weight of the rolling elements cannot prevent the rise in the bearing temperature.
By the way, the heat generated in the outer race during high speed rotation normally is radiated to the exterior through the housing. Since the heat generated in the inner race can be difficultly radiated from the rotary axis, the temperature of the inner race is higher than that of the outer race. Thus, if the outer race and the inner race are formed by the same material, and the temperature of the inner race is raised by heat generation, the inner race undergoes a great thermal expansion that reduces the bearing clearance from the initial value. The resulting preload is excessive, accelerating the heat generation. This phenomenon occurs in a vicious circle. Eventually, the bearing undergoes seizing that can lead to the destruction of the bearing.
From this standpoint of view, a rolling bearing has been proposed comprising an inner race formed by a material having a smaller linear expansion coefficient than the outer race material (see JP-B-7-30788 (The term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d)) (hereinafter referred to as xe2x80x9csecond conventional techniquexe2x80x9d). In accordance with the foregoing second conventional technique, the inner race is formed by a material having a smaller linear expansion coefficient than the outer race material. For example, the outer race may be formed by a high carbon chromium bearing steel (SUJ2) while the inner race may be formed by a stainless steel (SUS440C) or ceramic material. In this arrangement, even if the temperature of the inner race is higher than that of the outer race, the expansion of the inner race caused by the temperature difference between the inner race and the outer race can be inhibited. As a result, the variation of preload accompanying the change in the bearing clearance is reduced, making it possible to prevent the bearing from seizing.
A titanium alloy has a lighter weight and a higher strength than a steel material and a very excellent corrosion resistance among metallic materials and thus is expected to be a bearing material for use in special corrosive atmospheres such as those contaminated by water content, sea water, chemical, etc.
In a rolling bearing, however, a very great face pressure is applied to the portion at which the races and the rolling elements come in contact with each other. Thus, it is required for a rolling bearing to exhibit a high surface hardness. However, a titanium alloy which has been merely subjected to ordinary heat treatment such as solution treatment and aging cannot be provided with a desired surface hardness.
From this standpoint of view, a technique for enhancing the surface hardness of a titanium alloy by a predetermined surface treatment has been proposed (JP-B-61-2747) (hereinafter referred to as xe2x80x9cthird conventional techniquexe2x80x9d).
In the foregoing third conventional technique, a titanium alloy is subjected to gaseous nitriding or carburizing so that penetrating elements such as C, N and O are diffused in the form of solid solution therein, thereby securing the surface hardness required for the races.
In the foregoing first conventional technique, a ceramic material is used as bearing material. Thus, the bearing exhibits an extremely good corrosion resistance as compared with stainless steel. However, the first conventional technique is disadvantageous in that a ceramic material is inferior to stainless steel in strength or toughness and thus cannot be used without any trouble in atmospheres subject to great load. In particular, the use of ceramic material as the race material is undesirable from the standpoint of reliability of bearing.
Further, a ceramic material is remarkably inferior to metallic material in formability and grindability. Thus, if all the essential parts of a bearing are formed by a ceramic material, it disadvantageously adds to the production cost.
Moreover, a ceramic material has an extremely smaller linear expansion coefficient than a metallic material. Thus, the foregoing conventional technique has some disadvantages. For example, if the outer race is formed by the foregoing high carbon chromium steel (SUJ2) and the inner race is formed by a ceramic material, the difference in thermal expansion between the metallic rotary axis and the inner race made of ceramic material becomes too great when the temperature rises to relax the thermal expansion of the rotary axis, possibly cracking the inner race made of ceramic material and hence causing the destruction of the bearing.
On the other hand, if the outer race is formed by a high carbon chromium bearing steel (SUJ2) and the inner race is formed by a stainless steel (SUS440C), the change in the bearing clearance caused by the temperature rise can be minimized because the linear expansion coefficient of stainless steel is as small as 80% of that of high carbon chromium bearing steel. Further, since a stainless steel is a metallic material, the inner race made of stainless steel is considered to be insusceptible to cracking due to the difference in thermal expansion between the rotary axis and the inner race unlike the inner race made of ceramic material.
However, since the stainless steel used as inner race material has a higher density (higher specific gravity) than the ceramic material, the rise in the centrifugal force applied to the inner race cannot be neglected. In other words, since centrifugal force increases in proportion to mass and speed, the inner race expands due to the centrifugal force produced by rotation as the rotary speed increases. As a result, the bearing clearance is reduced, accelerating the heat generation.
The foregoing third conventional technique is disadvantageous in that the resulting surface hardness and depth of hardening differ greatly with the kind of penetrating elements to be incorporated in the form of solid solution by surface treatment. Further, some titanium alloys used have too low a strength in the core to fulfill a sufficient function as bearing.
In accordance with the third conventional technique, the surface hardness of the titanium alloy can be enhanced by diffusing penetrating elements in the titanium alloy in the form of solid solution. However, these penetrating elements can embrittle the titanium alloy, making it impossible to obtain a desired bearing life.
It is therefore an object of the present invention to provide a rolling bearing excellent in corrosion resistance, toughness and high rotary speed operation.
The foregoing and other objects of the present invention will become more apparent from the following detailed description and examples.
The objects are achieved by the following embodiments mainly.
(1) A rolling bearing comprising races composed of an outer race and an inner race and rolling elements which are provided between the outer race and the inner race such that the rolling elements rotate freely, wherein at least the inner race is made of a titanium alloy and the rolling elements are made of a corrosion-resistant material.
(2) The rolling bearing of item (1), wherein the titanium alloy is selected from the group consisting of xcex2 type titanium alloy and (xcex1+xcex2) type titanium alloy and the corrosion-resistant material is selected from the group consisting of ceramics and martensite stainless steel.
(3) The rolling bearing of item (1), wherein the surface hardness (Hv) of the finished raceway track on at least one race selected from the group consisting of the outer race and the inner race is not less than 600.
(4) The rolling bearing of item (1), wherein the surface of the finished raceway track on the at least one race comprises a mixture of xcex1 phase texture and xcex2 phase texture, the proportion of the xcex2 phase in the mixture being from 30 to 80 vol %.
(5) A method for producing a rolling bearing, which comprises preparing at least one race selected from the group consisting of an outer race and an inner race according to a method which comprises steps of:
(a) selecting at least one from the group consisting of xcex2 type titanium alloy and (xcex1+xcex2) type titanium alloy as a race material;
(b) heating and keeping said race material at the temperature falling within the range of xcex2 phase temperature of not lower than xcex2 phase transition point (xcex2-phase transus) to effect solution treatment such that the phase of the texture of said race material is converted to xcex2 phase;
(c) rapidly cooling said race material so that the texture of said race material normally stays in xcex2 single phase;
(d) subjecting said race material to plastic working (cold working) so that it is shaped as desired and given work strain, which enables formation of nuclei of xcex1 phase which is harder than xcex2 phase and the xcex1 phase to be finely deposited in xcex2 phase;
(e) subjecting said race material to aging at a predetermined temperature lower than xcex2 phase transition point, whereby nuclei of xcex1 phase are formed and grown and the xcex1 phase is finely deposited in xcex2 phase; and then
(f) machining said race material to a race.
(6) The method of item (5), wherein the percent plastic working at the step (d) is not less than 20%.
(7) The method of item (5), wherein the percent plastic working is from 5 to 30% and the surface of the raceway track is subjected to shot peening before aging.
(8) The method of item (7), wherein shot peening is effected after aging.